docs: def size(self):
      """The number of instances in the data. If the underlying data source changes,
         it may be outdated.
      """
      import tensorflow as tf
  
      if self._size is None:
        self._size = 0
        options = tf.python_io.TFRecordOptions(tf.python_io.TFRecordCompressionType.GZIP)
        for tfexample_file in self.files:
          self._size += sum(1 for x
                            in tf.python_io.tf_record_iterator(tfexample_file, options=options))
  
      return self._size